A radio relay apparatus has heretofore been widely used which relays data exchanged between a radio base station and a radio terminal. Such a radio terminal can communicate with the radio base station through the radio relay apparatus even if the radio terminal is located outside a cell (communication area) formed by the radio base station, or located in a peripheral portion of the cell (so-called a cell fringe).
Such a radio relay apparatus includes a first radio communication unit configured to perform radio communications with the radio base station, a second radio communication unit configured to perform radio communications with the radio terminal, and a control unit configured to control the first radio communication unit and the second radio communication unit.
Further, a time division duplexing (TDD) scheme is known as a scheme for achieving bidirectional communication in a radio communication system. According to the TDD scheme, each of communication frames includes, in a time-divided manner, a downlink period for transmitting data from the radio base station to the radio terminal and an uplink period for transmitting data from the radio terminal to the radio base station.
In the case of using the radio relay apparatus in a radio communication system employing the TDD scheme, in the downlink period, the first radio communication unit receives a radio signal from the radio base station and, at the same time, the second radio communication unit transmits a radio signal to the radio terminal. For this reason, the radio signal transmitted by the second radio communication unit interferes with the first radio communication unit. Likewise, in the uplink period, the second radio communication unit receives a radio signal from the radio terminal and, at the same time, the first radio communication unit transmits a radio signal to the radio base station. For this reason, the radio signal transmitted by the first radio communication unit interferes with the second radio communication unit.
To solve such a problem, there has been proposed a radio relay apparatus configured so that the period during which the second radio communication unit transmits the radio signal to the radio terminal is shifted from the downlink period to the uplink period and the period during which the second radio communication unit receives the radio signal from the radio terminal is shifted from the uplink period to the downlink period (see Patent Document 1).
The radio base station and the second radio communication unit of the radio relay apparatus transmit synchronization signals at predetermined timing in the downlink period, the synchronization signals being radio signals used by the radio terminal to establish synchronization. The radio terminal searches for the synchronization signal when starting radio communication, and establishes synchronization with the source of the synchronization signal when having received the synchronization signal successfully, thereby connecting to the source of the synchronization signal.
However, the radio relay apparatus described in Patent Document 1 shifts the period for transmitting the radio signal to the radio terminal to the uplink period. Therefore, the radio terminal located near the radio relay apparatus receives the synchronization signal (hereinafter referred to as the first synchronization signal) from the radio base station and the synchronization signal (hereinafter referred to as the second synchronization signal) from the radio relay apparatus at different timings.
The radio terminal located near the radio relay apparatus can perform radio communication in good communication conditions by connecting to the radio relay apparatus. However, when receiving the first and second synchronization signals at different timings, the radio terminal may be connected to the radio base station or may fail to establish synchronization due to the uncertainty of a synchronization destination. This leads to a problem that communication in good communication conditions cannot be performed.